Diaphragm valves contain a diaphragm in the form of a sheet of rubber or other suitable elastomer which is generally clamped along its edges between two portions of the valve casing and is adapted to close the valve by being pressed onto a valve seat in the form of a weir separating the zones of different pressures. Pressure is applied on the diaphragm sheet by means of a hand-wheel and a spindle which is rotated by the operator until he encounters sufficient resistance against his turning motion. The pressure applied is obviously not controlled, since every person will apply a torque that appears sufficient to his own senses, and it is a common experience that the diaphragm is usually pressed onto the seat with more than sufficient force, with the aim to ensure absolute tightness of the valve. As a consequence, the diaphram is often compressed beyond its elastic limit which leads to damage to the sheet surface, requiring its replacement. Since this expresses itself in both waste of time and costs and, in addition, in making the line inoperative during the repair period, it is the object of the present invention to prevent the diaphragm from being unduly compressed, however sufficiently to ensure complete closure of the valve.
It is an additional object of the invention to make provision for adjustment of the pressure exacted in relation to the thickness of the diaphragm and for compensating for certain wear of its surface.